The object of the invention is to create a ferrous material for friction bodies that has the known advantages of previous cast materials, such as heat resistance, low risk of fire cracking, acoustic damping and measured lubrication, is of lower density, less prone to corrosion and, while being harder, is more workable than materials produced in accordance with the prior art. The properties that such a material must have and the measures required to produce it result from the characterising parts of the claims.
Materials of the type according to the invention are particularly suitable for use in brake discs, brake drums or clutch discs in road or rail transport systems, but are generally usable anywhere that, for acceleration or deceleration operations, a transmission of power is required between two bodies, at least one of which performs a translational or rotational motion relative to the other body. The coefficients of friction between two friction surfaces are constant or variable according to the design.
Drum brakes, multiple-disc brakes and disc brakes are used in brakes for road or rail transport systems. The discs used in disc brakes are either solid or internally ventilated, depending on the thermal load. They are largely manufactured from cast iron conforming to DIN 15437 with lamellar graphite (e.g. GG 25) or spheroidal graphite (e.g. GGG 40, GGG 50 or GGG 60), cast steel GS 60, structure) steel St 52-3 or, less frequently, heat treatable steels C 45 or 42CrM04. The use of grey cast iron with lamellar graphite (GG 25) has proven particularly advantageous. The flat form of graphite present therein produces the desired high thermal conductivity. The free carbon required to form graphite is obtained by alloying the pig iron with silicon. In so doing, every effort should be made to keep the silicon content as low as possible since silicon reduces the thermal conductivity of iron. Otherwise unalloyed, high carbon cast iron materials have proven particularly suitable. Unalloyed cast iron results in fewer failures of brake discs due to fire cracking and dimensional changes due to warping than low alloy cast iron or cast steel. The braking behaviour is also the same or better. There is less wear, despite the reduced hardness, and the brake discs are less prone to drumming and rubbing. The thermal conductivity of a GG 15 MC derived from GG 25, for example, is less than 50 W/mK. This low-cost material is also easily machinable.
The material GG 20 is often used for brake discs. Its chemical composition is shown in Table 1 below.
A low Mo, Cr and Ni content may be added by alloying in order to increase the strength and stabilise the perlite. According to DIN 1690, GG 20 has a tensile strength of 200 to 300 N/mm2, which exceeds the tensile strength of at least 150 N/mm2 desired for use. The Brinell hardness of the material is 180 to 220 MB.
However, the disadvantage of all the previously known grey cast iron materials for friction bodies is that the elongation at break is too low. Alloys of grey cast iron with spheroidal graphite or cast steel or, less frequently, heat treatable steels are used for many applications, particularly if heating is uneven over the friction surface, i.e. so-called hot spots occur. This does not, however, have a positive effect on the other disadvantage, namely the relatively high susceptibility to corrosion of the material. Finally, the high density of friction bodies manufactured from the material described above, the fast abrasion, which is particularly noticeable when braking large masses such as trucks or rail vehicles, and the cost of expensive alloying elements should all be mentioned as further disadvantages.
Several approaches may be taken to improve the properties of today""s iron based friction body materials. Alloying elements may be considered in order to improve the corrosion properties. Expensive alloying materials cause an undue increase in the cost of materials, however. Weight can be reduced primarily through the use of a lighter alloy. If aluminum is used as the basic material, the corrosion behaviour is improved, but the wearing properties deteriorate drastically and the operating temperature is limited.
All the aforementioned disadvantages essentially result from the microstructure of the material, since the matrix consists of a mixed Fe/C-based crystal which is frequently ferritic or perlitic or martensitic. The unordered arrangement of the metal atoms is characteristic of such mixed crystals. The bonds therebetween are almost exclusively metallic. On the other hand, alloys with intermetallic compounds offer a good alternative. Intermetallic compounds have ordered crystal lattices with a high proportion of ionic or covalent bonds. Although they consist only of metallic elements, they have the properties of oxide, carbide or nitride ceramics, are characterised by their high melting temperatures and very high corrosion resistance. It therefore appeared conceivable that the properties of friction bodies could be improved substantially through the use of an intermetallic compound. The intermetallic compounds considered were those based on iron and aluminium, in particular, on account of the weight reduction which are also desired.
There were, however, considerable reservations concerning iron/aluminium compound since they were generally thought to be difficult to produce, brittle and difficult to machine. In this respect, reference is made to the information sheets from the Zentrale fur Gubverwendung (the central office for cast iron usage), sheet no. 1105/1 (9th edition 1968), for example, which still reflects the prior art today. In this information sheet, aluminum-alloyed cast irons with a low aluminium content (4 to 7% Al) are described as having low impact bending strength and low workability and those with a high aluminium content of 22 to 30% Al are also described as having low impact bending strength and even poorer workability. Parts for furnaces, apparatuses for sulphur distillation and for the manufacture of sodium sulphide, smelting crucibles, heating plates and resistor packs are mentioned as typical applications, but not structural components subject to mechanical stresses. It is noted at several points in the publication that although they have good scaling resistance, they have the disadvantages of being difficult to manufacture, frequently poorly workable and brittle.
In contrast to the disadvantages described above; and, however, the experiments on which the invention is based demonstrated that these disadvantages do not occur, which corresponded to the result of prior theoretical consideration. Considerably higher corrosion resistance was observed. The impact bending strength at least corresponded to that of conventional materials and the workability was even improved.